Process for the preparation of alkall metal cyanide solutions



Jan. 25, 1927. UNITED STATES PATENT oFFIcE.

xua'r' mmucn, or" rnaNxronr-oN-mnn-msm, GERMANY, as'smnon 'ro nonssmm a;

HASSLACHER CHEMICAL COMPANY, OF YORK, N. Y., A CORPORATION OF NEW YORK.

' rnocnss non run rnnrana'rrou or ALKALI METAL CYAIQDE sonu'r'rous.

No Drawing. Application filed December 22. 1925, Serial no. 77,128, andin Germany December 28, 1924.

This invention relates ,to the preparation of concentrated alkali metalcyanide solutions by the absorption of hydrocyanic acid b It is knownthat when hydrocyanic acid gas is passed into a solution, mixture oflime and an alkali metal sulphate an alkali metal cyanide solution willbe formed and calcium sulphate will be precipitated. All attempts to 4utilize this fact for the production of concentrated solutions met withan unexthe reaction can be carried outwithout meetpected difficulty sothat the process is of no commercial value. The difliculty encounteredwas the formation of an exceedingly voluminous precipitate. Thisprecipitate is so voluminous and the thickening of the solution causedthereby so great that effective stirring became impossible and theabsorption of the hydrocyamc acid which, further, was passed into thesolution was incomplete.

The cause of this troublesome precipitate,

ever, as this critical concentration is ex ceeded, the formation of, thetroublesome double salt becomes noticeable. i

I have now toimd conditions under/ which ing with thediflicultymentioned. When the critical concentration of alkali metalcyanide is reached or more advantageously still,

just before itis attained I keep the concen-' tration of the metalsulphate so low that't-he formationroi the double salt is-e itheravoided alto ther or that troublesome uantities ther-e0? concentrationof the alkali metal cyanide depends on various conditions: chiefly onthe amount of alkali metal sulphate resent, fur thermore on thetemperature an so forth. i

I have found for example that I can work safely (at ordin. presence ofthe tot -qluantity of lime which is requiredby the fina sodiumcyanidecon centration desired, when the total sodium ad ing *freshamounts are not formed. The critical I p also be added even before thetotal sodium sulphate has been converted-into sodium cya-" temperature)Y in the to 20% is so great upon the sodium sulphate.

which remains still uncovered, that the double sodium-calcium sulphateis salted out either by the sodium cyamde or by the sodi? um sulphate.If however the sodium sulphate' concentration at the beginning of thebatch exceeds a' proportion of 28 parts'of sodium sulphate in 100 partsof water, the sodium cyanide concentrations very soon become too high inrelation to the sodium sulphate solution still unchanged. The formationand se aration of the voluminous double sulphate egins fairly soon andat sodium cyanide concentrations which are considerably below 18-20%.

The operation, therefore, is carried into effect with advantage in sucha way that the batch is started with a solution-mixture which containsthe total lime-and about 28 parts of sodium sulphate in 100 parts ofwater. The hydrocyanic acid s is now passed through this mixture resting in the formation of sodium cyanide and the separation of ggpsum. Assoon as the total quantity of so umcyanide the process can be carried onby 0 either continuously oi intermittently in small portions. In adding'thesodium sulr sodium sulphate mm sulphate is converted into sodiphatecare has to be ftaken, that its "conversion into sodium cyanide proceedsequally quickly as the introduction of hydrocyanic Y acid or that itis-added in such quantitiesonly as will not lead to too hi h-aconcentras tion and consequent formatlon ofthe in-' soluble doublesulphate. v

Fresh quantities of sodium" sulphatemay nide. But in this case it isdiflicult to avoid the dan e'rous sodium sulphate concentratime an theprocess has to be watched far more carefully as when worklng' accordi'to the first'described method. zng The best, easiest and safestwayfhowever, F

" Ewqmple I.22Q liters of dilute solutions first amount has beencompletely utilized. In this case I proceed as follows:

After the sodium sulphate which has been primarily applied, has beencompletely utilized, the introduction of the hydrocyanic acid gas iscontinued regardless of the ab sence of any sodium sulphate. The hdrocyanic acid is absorbed by the lime whlch is present in great excessand forms with it calcium cyanide. After a certain quantity of thiscyanide of calcium has been formed, sodium sulphate is added whichreacts immediately with the calcium cyanide previously formed withformation of sodium cyanide and simultaneous deposition of calcium sul-'phate. The quantities of sodium sulphate to be added when working according'to this method are advantageously kept so small that there remainsalways an excess of calcium cyanide not reacted upon by the sodiumsulphate. In this way the presence of free sodium sulphate capable offorming the double salt with calcium sulphate is avoided altogether,when the process proceeds. After the addition of these amounts of sodiumsulphate again calcium cyanide is formed by the continued introductionof the .hydrocyanic a id gas; this is again converted into so iumcyanide after a certain time by the addition of a fresh measuredquantity of sodium sulphate subject to the same precautions. At the endof the process the current of hydrocyanic acid gas is stopped and thecalcium cyanide still present is .converted into sodium cyanide by acorresponding amount of sodium sulphate.

When working according to my new method I obtain easily solutions ofvery high sodium'cyanide concentration, for instance solutions of 45%without encountering any of the difficulties previously described. Theamount of lime required to react with the sulphuric acid of the sodiumsulphateto be added, can all be introduced proceeds, but care has to betaken that it is always present in excess in relation to the sodiumsulphate.

It could not be foreseen that the reaction would proceed smoothly andwithout anv trouble as it was known that calcium cyanide very easilydecomposes. There was there-. fore the danger that this decompositionxwould cause considerable loss of'hydrocyanic acid. I found to mysurprise,"howev'er, that this did not happen as the sodium cyanide beingpresent in higher concentration ap- 'parently protects the calciumcyanide against decomposition. I

My invention; shall be described by an 4 example taken from actualpractical work, where the wash waters of a'previous batch are-taken "tostart a' new batch.

at the start of a particular batch or itmay V be added in small amountsas the reaction from previous washings containing 11% NaQN and 10% N aSO are mixed with 100 kilogrammes of lime (with 73% Ca (0H),) and 30kgrms. of sodium sulphate. Through this solution-mixture hydrocyanicacid gas is bubbled with stirring. After about 20 minutes the whole ofthe sodium sulphate present is already completely converted. Theintroduction of the gas into the mixture is continued Without any'moresodium sulphate being added in the meantime. After 15 more m nutes havepassed 30 kgrms. of sodium sulphate are added in order to convertthecalcium cyanide previously formed. When this has been done 25 kgrms. ofsodium sulphate are introduced at the end of every 20 minutes untilanother 75 kgrms. of sodium sulphatenre applied bringing the total ofsodium sulphate used up to 135 kgrms. The temperature of the mixture,which was 19 centigrade at the beginning rises during the course of thereaction to about 40 (1.; it is maintained there but is allowed to fallusually towards the end of the process to about 37? C. At the 'end ofthe hydrocyanic acid addition the stirring is continued for a short timeand then the small excess of calcium cyanide is converted into sodiumcyanide by the addition of the corresponding quantity of sodiumsulphate. The solution is subsequently separated by filtration from thecalcium sulphate precipitate. The filtrate contains 36% sodium cyanide.The precipitate is freed from any sodium cyanide b washing and thewashings are utilized or a new batch.

Furthermore I have found that the for mation of the double sulphate ofcalcium and an alkali metal is favored by higher temperaturesbut that itis not stable at temperatures ,which do not exceed 15 centigrade. Thecritical temperature depends to a certain.vv extent upon the generalworking conditions, for instance upon the sodium sulphate and sodiumcyanide concentrations. By operation at a temperature not exceedingtotal of the lime necessary for obtaining the desired final sodiumcyanide concentration. fWhen using this method it has been foundadvantageous to raise the temperature of the batch at the end of theabsorption -rocess in order to bring any precipitated so ium cyanide'back into solution.v

12-15 centigrade the deposition or the .for-v They may even con- .Thus Ican adapt myfprocess to any situation by employing a combination of theabove methods of avoiding the formation of the double alkali-calciumsalt or employing them separately.

As example of the second method the following is given: H

Example II.32,7 parts by weight of 90% calcium hydroxide and,52,3Ipartsby weight of 99% sodium sulphate are added to 100 parts byweight of a solution containing 194 grms. of sodium cyanide per liter.(This solution coming from a dilute second and third wash water). 18,9parts by weight of hydrocyanic acid gas are now bubbled in with stirringthroughout a period of one hour The rate of gas addition being .suchthat the temperature of the mixture does not rise above 15 C. duringthefirst minutes; in the last 15 minutes the temperature isforced up toabout40' C.

The solution obtained on filtering contains about 13 liter. What I claimis:

1. Process for the preparation of a concentrated solution ofan alkalimetal cyanide which consists in reacting hydrocyanic acid with anaqueous mixture of alkali metal sulphate and calcium hydroxide thealkali metal sulphate being present-in ,an amount insuflicient to form a'double sulphate with the calcium sulphate formed in the reaction. 1

grams of sodium cyanide per 2. Process tor the preparation of aconcentratedsolution of an alkali metal cyanide which consists in theintroduction of hydrocyanic acid in an a ueous mixture of lime andalkali metal sulp ate, adding fresh quantities of alkali metal sulphatewhen or before such alkali metal cyanide concentra cause, corres onamounts-then being added before, when ,or

after the previous amounts have been con verted intosodium cyanide andre lating its quantity in such-a way, that by ti: addition suchconcentrations in the solution of alkali metal sulphate are avoided aswill ing to the increasing alkali metal cyani e concentration, theseparation of double sul hate of lime and alkali metal.

4. .Process or the prefiaration of -concentrated solution of an a limeand alkali metal sulphate, adding furali metal cyanide which cons ts inthe introduction of hydrocyamc acid m an aqueous mixture of lime and aall metal sulphate, adding fresh al-' kali metal sulphate aduall and insuch quantities that the su phate a ded is utilized for the formation ofcyanide before it has the possibility of forming a double sulphate withthe gypsum formed in the reaction, the addition of the alkali metalsulphate taking place when or before such alkali metal cyanideconcentrations are attained as give rise to the separation of doublesalts of alkali metal sulphate and calcium sulphate.

5. Process for the preparation of concentrated solution of an alkalimetal cyanide which consists in the introduction of hydrocyanic acid inan a ueous mixture of lime and alkali metal su phate, adding furtheralkali metal sulphate when orbefore such alkali metal cyanideconcentrations are attained as cause the precipitation of the doublesalts of alkali lmetal sulphate and calcium sulphate, the furtheraddition of the alkali metal'sulphate taking place only after a certainamount of calcium cyanide has been f ormed.

6. Process for the preparation of a concentrated solution or an alkalimetal cyanide which consists in the introduction of hydrocyanic acid inan aqueous mixture of lime and alkali metal sulphate, adding furtheralkali metal sulphate when or before such alkali metal cyanideconcentrations are attained as cause the precipitation of the doublesalts of alkali metal sul hate and calcium sulphate, the further adition of the alkali metal sulphate taking place only after -a certainamount of calcium cyanide has been formed and the quantity added beingkept below that which is necessary for the complete conversion into thealkali metal cyanide of thetotal calcium cyanide present.

'7. Process for the preparationof a concentrated solution of an alkalimetal cyanide which consists in the introduction of hydrocyanic acid inan aqueous mixture of ther alkali metal sulphate when or fore suchalkali metal cyanide concentrations are attained as cause theprecipitation of the double salts of "alkali metal sulphate and calciumsulphate, the, further quantities of 4 alkali metal sulphate beingintroduced in the form of several portions to be added after certainintervals in order to convert into alkali metal-cyanide the calciumcyanide which in the meantime has been formed.

8. Process for the preparation of a concentrated solution of an alkalimetal cyanide which consists in reacting hydrocyanic acid with anaqueous mixture of calcium hydroxide and an alkali metal sulphate applyoy in. a

ing the alkali metal sulphate primari "such a quantityionlythat thealkali metal centrated solution of an alkali metal cyanide whichconsists in reacting hydrocyamc acid with an aqueous mixture of calciumhydroxide and-an alkali metal sulphate, passing the l hydrocyanic acidinto a solution-mixture of lime and alkali metal sulphate which does notcontain more alkali metal sulphate than.

is necessary for the formation of a alkali metal cyanide solution, untilthe alkali metal sulphate has been substantially used .up, and addingsubsequently further amounts of alkali metal sulphate with the provisionthat such alkali metal sulphate concentrations are avoided as give riseto the formation and separation of the double sulphate.

' 10; Process for the preparation of a concentrated solution of sodiumcyanide which consists in reacting hydrocyanic acid with an aqueousmixture of sodium sulphate, and lime, the sodium sulphate being presentin an amount insufiicient to form a double sulphate with the calciumsulphate formed in the reaction taking. care that the lime present isalways in excess over the sodium sulphate.

11. Process for the preparation of a concentrated solution of an alkalimetal cyanide which consists in reacting hydrocyanic acid with anaqueous mixture of alkali metal sulphate and calcium hydroxide at atemperature below that at which the formation and separation of thedouble calcium alkali metal sulphate becomes possible.

12. Process for the preparation of acoucentrated solution ofan alkalimetal cyanide which consists in reacting hydrocyanlc acid with anaqueous mixture of alkali metal sulphate and calcium hydroxide at atemperature of about 15 centigrade.

Signed at Frankfort-on-the-Main, Germany, this 2d day ofDecember A. D.,1925.

KURT ANDRICH.

